Is High Efficiency green energy a reality for small wind turbines?
With the world focus on renewable energy sources, going green is a political statement that can gain a company some kudos, but are they really “green” in the renewable sense of the word?
Wind Turbine Technology
For the wind industry, the challenge is to design wind turbines to harness the wind energy and generate electricity efficiently.
Wind turbines have increased in size from 25 kW to 2500 kW and greater, with the cost of the energy generated reduced by a factor >5 as the industry moved from the “green” fringe activity, to an acknowledged power generation industry.
The engineering base has developed to match machine size and volume, but many technical challenges remain to meet the energy goals outlined in various government directives.
Evolution of Wind Turbine Technology
The concept of a wind driven rotor is ancient, and electric motors for generation of electricity have been widely used in domestic and commercial areas. A wind turbine may look simple, but to produce an efficient, green, wind turbine that meets frequency, voltage, harmonic content, specifications for electricity generation operating unattended, maximise the available cut in wind speeds, which on exploitable sites, may range from <3 m/s to >11 m/s with turbulence and have effective gust management to survive gusts up to 70 m/s to compete economically with other energy sources, is a challenge.
A draft of the European Union’s plans for wind and solar energy has recently been leaked (Reuters), revealing EU intentions to quickly build wind turbines and solar terminals throughout member states.
Italy is to increase its on-shore wind farm capacity by 230 percent, Ireland by 130 percent, Spain by 74 percent, and Germany by 30 percent.
Design Drivers for Modern Technology
The main design drivers for current large capacity wind technology are:
Low and high wind sites; Grid compatibility; Acoustic performance; Aerodynamic performance; Visual impact; Low mass nacelle arrangements; Rotor technology with advanced composite engineering, design for offshore. erection and maintenance.
The return of development interest to new production lines for the size ranges most relevant to the land-based market, from 800 kW up to about 3 MW. Of the other main drivers, larger rotor diameters (in relation to rated output power) have been introduced in order to enhance exploitation of low wind speed sites.
But is this design drive suitable for the lower generation capacity markets?
Small wind turbines.
Small wind turbines (SWTs) are used in two main areas:
‘Autonomous’ electrical systems not connected to any larger electrical system.
‘Distributed generation’, systems with small generators connected to a larger public distribution network.
The technology of small wind turbines is different from that used in large wind turbines, and affects all of the subsystems, the control and electrical systems, and significantly the design of the rotor and the generator. Most of the SWTs existing on the market are machines that have developed in an almost ‘hand crafted’ way, with lower maturity compared to that achieved by large wind turbines. A large amount of development has been carried out on developing small rotor configuration of the type shown in fig 1, but limited work on the generator until now.
Autonomous and distributed generation using small wind turbines are attractive as the price of conventional electricity and fossil fuels is increasing in the developed world which has a high interest in green energy and the cost is a lower barrier to installation..
In many developing countries, millions of people live without access to electricity, SWT have a major impact on the quality of life enabling access to remote services and the internet through mobile phone technology.
However, the state of the art for small wind turbines is far from technological maturity and economical competitiveness.
SWTs have great potential to produce reliable machines. IEC standards do exist for SWTs IEC61400-2 for design requirements however, development of the standards is required to produce more appropriate and simpler ways to display the results obtained, to the end users.
In developed countries the market for SWT is promising for grid and off-grid applications due to promotion policies such as capital cost buy-down, feed-in tariffs, net metering, and for developing countries because of the reduction in manufacturing costs and the increasing energy requierments.
Classification of SWT from 0 watts to 100kW generation
Rated power (kW) Rotor swept area (m2) Sub-category
Prated < 1 kW A < 4.9 m2 Pico wind
1 kW < Prated< 7 kW A < 40 m2 Micro wind
7 kW < Prated< 50 kW A < 200 m2 Mini wind
50 kW < Prated< 100 kW A < 300 m2 (No clear definition adopted yet)
Direct drive, a new technology for SWT
Direct drive wind generators simplify the nacelle system, increase reliability, increase efficiency and avoid gearbox issues. The trend towards direct drive systems in the large wind generator systems has been evident for some years in producing technology that is lighter or more cost-effective than the conventional geared drive trains. Although these developments continue, direct drive turbines have not, as yet, had a sizeable market share. Most direct drive designs are based on permanent magnets generator (PMG) technology, using high-strength Neodymium magnets.
Magnetic Motori, a well know motor designer and manufacturer based in Italy, set out to develop a direct drive solution to meet the needs of a reliable and efficient power generation permanent magnet motor/generator for renewable energy applications and high efficiency drive systems in SWT.
It is a fact of wind generation that the power train components of a wind turbine, regardless of the generation capacity, are subject to highly irregular loading input from turbulent wind conditions.
Fatigue cycles experienced by the structural components can be significantly greater than for other rotating machines. Consider that a large, modern wind turbine operates about 13 years in a design life of 20 years virtually unattended as does a SWT.
The HTQ was designed with high overload capacity to usefully exploit the momentary increase in wind speeds, (turbulence and gust management), and wave variations seen in wave power (wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents), applications.
This leads to a high conversion performance considering that this is highly dependent on the speed itself.
Size & Cooling
The presence of a flow of outside air or the availability of water, in wind and wave power generation, was designed to be used as the cooling medium. An efficient cooling system allows the size of the machines to be smaller with a better power to weight ratio, and reduces the related costs.
The gearless solution (commonly called gearless direct-drive) is justified by economic returns resulting from increased efficiency, reduction of drive gear components and improved mechanical reliability.
The HTQ high efficiency series is, internally, literally green due to the colour of the special enamel used to coat the copper used for the windings. Starting from basic principles and understanding the real world end application, Magnetic identified key problems associated with renewable power generation and the efficient conversion needed to supply the power to the local electricity supply system, size, cooling, overload capacity, sealing protection, and electrical reliability.
With motor sealing protection greater than or equal to IP54 is linked with an extended range of torque / power ratios available.
To usefully extract the electrical energy generated, the HTQ series use multi pole (up to 22) rare earth neodymium iron boron magnets, the strongest and most affordable type of rare-earth magnet, having the highest coercivity and size ratio. Fitted into the rotor in a propriety method and completely enclosed in an epoxy protection coating, the mechanic and magnetic circuits are defined, controlled, and stable under all operating conditions.
And the reason for the green coating of the copper wire?
The potential acceleration and deceleration conditions experienced in renewable power generation rotor can create very high rates of change of voltage, (dV/dT), back emf, high frequency and high amplitude harmonics etc. In the stator windings, and in normal enamels these conditions can electrically breakdown the enamel, leading to premature electrical failure.
The proprietary copper coating used by magnetic in the HTQ series has high dV/dT withstand capability, high frequency permittivity/dielectric isolation characteristics, linked with a high temperature rating class H for inverter applications, with double vacuum impregnation through autoclave with class H resins.
These characteristics ensure the motors have a very high resistance to transient spikes, high frequencies, and short rise time pulses.
And used as a motor, the same properties are required for inverter drive systems.
And the colour happens to be green.
Added Value Magnetic R&D
Market growths rates for wind generation are in the same range as those of high-tech technologies (internet, mobile phones and so on) with most of the top ten manufacturers being European.
There is a misconception to consider wind energy as a mature technology and R&D efforts is not required. Magnetic are continuing the development of the HTQ range and as a result, power generation capability up to 50kW are now available as standard with larger powers in preliminary design.
The European target is 20 per cent of energy production from renewable sources. In its recently published Strategic Research Agenda, the European wind energy platform, TPWind, proposed an ambitious, and feasible, vision for Europe. In this vision, 300 GW of wind energy capacity would be delivered by 2030, representing up to 28 per cent of EU electricity consumption. To implement this, an average 10 to 15 GW of additional renewable green capacity must be manufactured, delivered and implemented every year in Europe. This is equivalent to more than 20 large turbines of 3 MW being installed each working day, or a large number of SWT.
Thus in the use of the turbulent conditions and overcoming the severity of the fatigue environment, small wind turbine technology has a unique technical identity and unique R&D demands that magnetic has overcome with the HTQ series.
Going green is not an easy option, but the benefits to the industry the HTQ series can offer, in truly green renewable energy power generation, have been demonstrated. The conversion efficiencies greater than 80% are now posiable with the HTQ range. With cut in wind speeds and rotor speed close to zero, (depending on the rotor design) the HTQ is capable of generating useful amounts of energy from a wide range of climatic conditions.
Magnetic believes that high efficiency green energy is a reality when you use the HTQ series.
Light weight aluminium construction, hollow and shaft versions, available in a wider range of torque/RPM combinations, the Magnetic HTQ series fits most applications. If you need further technical information to find a suitable HTQ generator/motor for your application or a larger, that is up to1250mm diameter, contact:
Alldrives & Controls the UK representative for Magnetics Italy.